The present invention relates to a thermal printing system, in particular, relates to such a system which improves the operating speed.
A thermal recording system utilizes a thermal head having a series of aligned heat-generating resistor elements. An electric current is applied to each of the heat-generating resistor elements in accordance with the black or white information of a picture cell to be recorded, so that the Joule heat thus generated in the resistor elements is transferred to a heat-sensitive treated thermal paper in close contact with the thermal head for color formation. Such a heat sensitive recording system has been used in fascimile receiver and/or a computer terminal device and the like. An example of the thermal head and the apparatus for supplying the electric power to said thermal head has been shown in the U.S. Pat. No. 3,609,294.
The heat-sensitive color formation theoretically requires a comparatively long time (about 1.5 to 10 mS) for recording, so that, when high-speed recording is required a simultaneous multi-dot recording system which simultaneously records a plurality of dots is used.
The recording by a thermal printing head is carried out by the dot matrix process which generates the desired pattern by heating some thermal head elements selectively synchronized with the paper transfer. The information for selecting particular thermal elements for each desired pattern is usually stored in an integrated circuit (IC) memory.
The concentration or the color density of a thermal paper depends upon many factors, among which the electric power applied to thermal elements and the temperature of a thermal paper just before a thermal paper is heated are the most important factors. That is to say, when the temperature of a thermal paper is low, high power must be applied to a thermal element to obtain the desired density, while when the temperature of a thermal paper is high, less power is enough to provide the desired concentration. According to the particular example, a thermal element is heated by an electric pulse having the pedetermined amplitude and the pulse width of 1.5 milisecond to provide the desired density of color change of a thermal paper. The temperature of the heated thermal element and the heated portion of a thermal paper return to the normal temperature after 10 miliseconds from the time of heating. The duration of that 10 mS is called as a heat storage duration. Therefore, the period for heating a thermal element must be longer than that heat storage duration. If a thermal element is heated within the heat storage duration, the color change would be too dark. Accordingly, a prior thermal printing system takes the period of heating thermal elements longer than the heat storage duration in order to provide the desired color density.
However, said prior thermal printing system has the disadvantage that the printing speed is rather slow, since the period of heating thermal elements is long. Thus, a high speed printer which can print a line in the period of 1.5 to 2.0 milisecond has been almost impossible for a prior thermal printing system.
Further, when a printer is utilized as a facsimile receiver, which uses an information compression for high speed transmission, the printing speed must be variable depending upon the reception rate of the information, and when the reception rate exceeds the printing speed of the receiver, the information can not be printed.